Research Projects in the Mediterranean Area

Functional analysis of Mediterranean plant water relations and carbon fixation through carbon and oxygen stable isotopes
Invasive species in Mediterranean dune ecosystems

Functional analysis of Mediterranean plant water relations and carbon fixation through carbon and oxygen stable isotope

This project comprises a new approach to study plant water relations and limiting processes for carbon fixation under both, natural and controlled conditions, through the combinations of carbon and oxygen isotope discrimination in Mediterranean species of different functional groups. This project is conducted in collaboration with Dr. Cristina Máguas (cmaguas@fc.ul.pt)from the CEBV, university of Lisbon  and was started within the  NETCARB framework.

Stable isotopes - Theory

Stable isotopes are a powerful research tool in environmental sciences and their use in ecophysiological and ecosystem research is increasing. The discrimination against the heavier carbon isotope (13C) in plants has been used to model photosynthetic limitation and carboxylation pathway and can reveal information at different scales such as the leaf internal resistances, long-term analysis of canopy water-use efficiency, and ecosystem carbon fluxes and evaluation of changes in global CO2 supply. However, only recently stable isotopes at natural abundance levels have been introduced to plant water relations studies. While 13C is depleted during photosynthesis, leaf water is enriched in D or 18O during transpiration, since diffusion and enzymatic incorporation in general favours the lighter isotope. Variations in the natural abundance of stable isotopes of water, i.e. D/H and 18O/16O provide a natural tracer of water movement within plants as well as between plants and the environment. The 18O/16O content of gaseous oxygen has been directly related to the leaf-air vapour pressure deficit in relation to gas exchange, plant water status and soil-water.

Water use efficiency of  Mediterranean species

Regulation of water use efficiency (WUE) is particularly important in Mediterranean ecosystems, where plants are periodically exposed to severe water stress. The interpretation of carbon isotope discrimination (D13C) as a measure of WUE is difficult in evergreen species due to leaf longevity or coherent changes in stomatal conductance and photosynthetic capacity during drought. One of the objective was the evaluation of oxygen isotope ratio (δ18O) of bulk leaf material as a monitor of the evaporative conditions, to allow an improved analysis of seasonal changes of WUE.

These effects are analysed in 11 species of a Mediterranean plant community at the Serra da Arrabida, SW-Portugal, which comprise different functional groups regarding their drought adaptation mechanisms. On of the aims was to evaluate the usefulness of additional oxygen isotope analysis of the bulk leaf material to distinguish different strategies (Werner et al. 2001c).

Serra da Arrabida (Portugal)          Macchia

Mediterranean species have developed different strategies to cope with the strong seasonal changes in water availability. Woody species may be separated into different functional groups regarding their drought tolerance or avoiding mechanisms (see mediterranean Project, Werner et al. 1999, 2002). Shallow rooted semi-deciduous species (e.g. Cistus spp.) avoid summer drought by enhanced reduction of their transpirational surface, but recover rapidly after the first autumn rains. Deeper rooted evergreen sclerophylls (e.g. Quercus coccifera) have developed tap roots that may reach the lower water table even during summer drought. These species may utilize different water sources with different efficiencies for photosynthetic carbon fixation, which was analysed under field conditions.

Water sources

The oxygen isotope signal is influenced by the water sources of the plants (i.e.  precipitation, soil water, runoff and ground water). Soil water, runoff and groundwater differ significantly in their isotopic signature from rain due to fractionation occurring during physical processes such as evaporation, condensation and infiltration into the soil. It has been suggested that isotopic analysis of xylem sap may provide a signature of the source of soil moisture that a plant is using, since no fractionation occurs during water uptake by plant roots. Hence, root water, stem water and xylem water that have not been subjected to evaporative or metabolic fractionation will reflect the source or sources of water used by any particular species. 

To determine which sources of water are used by individual plants and different functional groups, the stable isotope composition of the water sources (precipitation and ground water) are collected from various sampling sites A comparison between stem water or xylem-water isotopic composition and the isotopic composition of the environmental water will be conducted.

Controlled drought experiment

The effects of different structural and functional adaptations on WUE are difficult to analysis under field conditions, where adaptation to drought occurs in a concomitant manner. To separate the effects of structural and functional adaptations on WUE, these patterns were further analysed in an experimental approach at the Lisbon University  with three species of different WUE´s through measurements of δ13C and δ18O of leaf dry matter, gas exchange and chlorophyll fluorescence, as well as phenological, structural and growth parameters. The three selected species represented three functional groups. It will be particularly important to link these structural and functional characteristics with the typical “spender” and “saver” strategy when subjected to a slow drying treatment. It has become evident that a wide range of external factors influence leaf D, as for example vapour pressure, temperature, light, water and nutrient supply. Additionally, life form, growth strategy or phenology should all be considered important determinants of D (Unger et al. 2001).

All isotope analysis were performed at the ICAT Stable Isotope Laboratory (LIE), Lisbon, using standard mass spectrometric techniques.

This first results indicate the advantage of combined use of carbon and oxygen isotopes of leaf dry matter for the interpretation of WUE and drought tolerance strategies, especially in long-lived evergreen species (Werner et al. 2001c).

Invasive species in Mediterranean dune ecosystems

 In Collaboration with Dr. Cristina Máguas and Prof. Otília Correia, CEBV, University of Lisbon

Troia undisturbed site   Troia sand dune system

Sand dunes are habitats of great nature conservation interest, with a very characteristic and rich flora. Given the importance of dune ecosystems, an environmental priority should be to protect the remaining sand dunes from destruction and degradation, and to manage these ecosystems in a sustainable way, which requires a profound scientific knowledge. Species like Acacia sp. have been introduced in the past, with the objective to stabilize dunes. However, some of these species became dominant, reducing native species density and biodiversity, causing serious ecological problems. This kind of manipulations lacked scientific background.

We are collaborating in the Portuguese project (coordinated by C. Máguas) on “Competition ability and resource dynamics in costal dune ecosystems: invasive vs. native species” which addresses these kind of problems. The project aims to understand the competition abilities of plants in well preserved and disturbed dune ecosystems, focussing on the interaction of invasive and native species. In particular we want to determine the competitiveness of both invasive and native species in relation to water and nutrient availability. The understanding of patterns of plant resource acquisition and plant ability to exploit resource availability are crucial for the interpretation of community dynamics and plant competition abilities. The project focuses on:

At the university of Bielefeld we are investigating these aspects under controlled conditions.